Most polymers, even those having functional or hydrolyzable groups thereon have surfaces which are too inert to enable them to receive and adhere to the thin coating of electrolessly deposited copper or nickel which are the substrates for electrolytic deposition of thicker metallic coatings such as those required for decorative and functional purposes such as EMI shielding, are usually made of ABS. Originally, mechanical means of micro-roughening these surfaces were used. These proved to be unsatisfactory and chemical methods were introduced. A satisfactory chemical mode is of sufficient vigour to change the nature of the surface layer but not enough to cause substantial structural deterioration.
The original chemical methods employed various procedures which were based on the use of chromic acid or some form of chromium ion in conjunction with a strong acid such as sulfuric acid. Environmental requirements now preclude the use of chromium due to its envirotoxic nature.
Among the alternate approaches may be mentioned that of Wiggins U.S. Pat. No. 4,415,406 who employs very strong sulfuric acid in the presence of formic and acetic acids, which also, due to their high vapor pressure have environmental problems in the work place. This process is not operative in the absence of the alkanoic acid component.
Another approach is an older one of Heyman U.S. Pat. No. 3,553,085. While Heyman uses chromic acid in some of his examples he also discloses the use of strong sulfuric acid in the presence of a combination of noble metal ions and oxidants. This disclosure alleges utility over a wide range of conditions but has certain operating disadvantages. The core of the process resides in depositing into the surface of the polymer, the ions of a noble metal, which are then reduced to the metallic state, forming an acceptable surface for the electroless deposition of nickel or copper.
The disclosure claims that as low a concentration of sulfuric as 50% is operative. The working examples which exclude chromium, however, utilize only concentrated sulfuric acid.
These examples do enable electroless deposition, but at a price of general surface damage which is not acceptable in modern commercial conditions. The formaldehyde in the electroless copper baths, will enable electroless plating to occur, however the adhesion of copper is not at a commercially satisfactory level. Electroless nickel plating will not occur. Sodium hypophosphite will apparently not reduce the silver ions to metal. The now conventional Pd.sup.0 bath will not deposit on surfaces treated at these high acid concentrations.
A possible reason may be that the process does not convert the polymer surface to a satisfactory level of hydrophilicity. Notwithstanding the allegations of the disclosure, when the sulfuric acid is less than 80%, an insufficient amount of noble metal is deposited to enable the desired electroless deposition to take place by the conditions disclosed by Heyman.
It is now well known to provide the nobel metal layer by dipping the micro-roughened i.e., hydrophilic surface into a colloidal suspension of palladium, commonly known as a Pd.sup.0 bath (see Wiggins, supra, col 4).
It would be desirable to obtain the desired micro-roughening without use of chromium or other problematic prior art methods to enable the direct deposition on a platable, i.e., nonhydrophobic surface, of the Pd.sup.0.